Various kinds of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of which have already been put into practical use. A widely employed precursor is a light-sensitive material having a photoconductive layer comprising photoconductive particles such as zinc oxide particles and a binder resin provided on a conductive support. A highly lipophilic toner image is subsequently formed on the photoconductive layer surface by an ordinary electrophotographic process. The surface of the photoconductive layer having the toner image is then treated with an oil-desensitizing solution, called an etching solution, to selectively render the non-image areas hydrophilic thereby producing an offset printing plate.
In order to obtain satisfactory prints, an offset printing plate precursor must faithfully reproduce an original on the surface thereof; the surface of the light-sensitive material should have a high affinity for an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic and, at the same time, should be water resistant. When used as printing plate, the photoconductive layer having a toner image formed thereon should not come off during printing, and should be well receptive to dampening water so that the non-image areas can remain sufficiently hydrophilic to be free from stains, even after a large number of prints have been reproduced from the plate.
These properties are affected by the proportion of binder resin to zinc oxide in the photoconductive layer as already known. Specifically, when the proportion of binder resin to zinc oxide particles in the photoconductive layer is decreased, the oil-desensitivity of the photoconductive layer surface is enhanced and background stains are decreased. However, the internal cohesive force and mechanical strength of the photoconductive layer itself is lowered, resulting in the deterioration of the printing durability. On the contrary, when the proportion of resin binder is increased, the background stains are increased although the printing durability is heightened. Background stains are related to the oil-desensitivity of the photoconductive layer surface. Not only does the ratio of binder resin to zinc oxide in the photoconductive layer influence the oil-desensitivity of the photoconductive layer surface, but it has become apparent that the oil-desensitivity also depends greatly on the kind of the binder resin employed.
With respect to the offset master, the background stain resulting from insufficiency in oil-desensitization is a particularly serious problem. For the purpose of solving this problem, various binder resins for zinc oxide have been developed for improving the oil-desensitivity. Resins having an effect on improvement in oil-desensitivity of the photoconductive layer include those as follows: JP-B-50-31011 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses the combination of a resin which has a weight average molecular weight of from 1.8.times.10.sup.4 to 1.times.10.sup.5 and a glass transition point (Tg) of from 10.degree. C. to 80.degree. C. and which is prepared by copolymerizing a (meth)acrylate monomer and another monomer in the presence of fumaric acid, with a copolymer prepared from a (meth)acrylate monomer and a monomer other than fumaric acid; JP-A-53-54027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a terpolymer comprising a (meth)acrylic acid ester unit having a substituent which contains a carboxylic acid group apart from the ester linkage by at least 7 atoms; JP-A-54-20735 and JP-A-57-202544 disclose a tetra- or penta-polymer comprising an acrylic acid unit and a hydroxyethyl (meth)acrylate unit; and JP-A-58-68046 discloses a tercopolymer comprising a (meth)acrylic acid ester unit having an alkyl group containing from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group.
However, even with the practical use of the above-described resins which are described to enhance oil-desensitivity, the resulting offset masters are still insufficient in resistance to background stains and printing durability.
The lithographic printing plate precursor utilizing a photoconductive zinc oxide is rendered its surface hydrophilic upon a chemical treatment of zinc oxide with an oil-desensitizing solution under an acidic condition as well known in the art. However, the oil-desensitizing solution which has good oil-desensitivity is limited to that containing a ferrocyanide as the main component.
As a result, there are various restrictions and problems encountered in that a method of treating waste fluid of the oil-desensitizing solution containing a ferrocyanide as the main component is needed, in that since it is necessary to maintain an acidic condition during printing, a number of prints obtainable remarkably decreases (i.e., degradation of printing durability), when neutral paper is employed for printing, and in that because the principle of oil-desensitization is based on the generation of hydrophilic substance upon a chelating reaction, the oil-desensitizing solution tends to interact with polyvalent metal ions contained in a color ink during printing so that unusual emulsification of ink occurs and consequently, a number of prints obtainable decreases particularly in case of color printing.
In order to reduce or solve these problems, there has been developed a techinque for providing hydrophilicity to non-image areas by means of rendering a binder resin of the photoconductive layer hydrophilic upon a chemical reaction treatment. For instance, resins of the type which contain functional groups capable of producing hydrophilic groups through decomposition have been investigated on an aptitude for the resin binder. For example, the resins containing functional groups capable of producing hydroxy groups by decomposition are disclosed in JP-A-62-195684, JP-A-62-210475 and JP-A-62-210476, those containing functional groups capable of producing carboxy groups through decomposition are disclosed in JP-A-62-212669, JP-A-62-286064 and JP-A-1-63977, and those containing functional groups capable of producing a sulfo group or a phosphono group through decomposition are disclosed in JP-A-63-260439,-JP-A-1-70767.
Further, an improvement in a composition for a photoconductive layer has been investigated in which resin grains containing a polymer component capable of forming a carboxy group, a sulfo group, a phosphono group or a hydroxy group through decomposition are incorporated into the photoconductive layer as described, for example, in JP-A-1-261658, JP-A-1-284856 and JP-A-1-287571. The printing plates prepared using these techniques certainly exhibit improved water retentivity as compared with conventional plates.